Liquid crystal phases which form a helix structure with a given sense of rotation are increasingly required for liquid crystal displays. Thus, such materials are required, for example, for the Schadt-Helfrich effect in order to avoid the undesirable effect of "reverse twist" (E. Guyon and W. Urbach in "Nonemissive Electrooptic Displays", published by A. R. Kmetz, F. K. von Willisen, Plenum Press, New York-London, 1976, page 127), for the cholesteric-nematic phase transition effect and for bistability effects.
An important problem here is the production of a suitable temperature function of the pitch of the helix, which depends on the particular electrooptical effect and its specific design.
For liquid crystal display elements based on the twisted nematic cell, for example, a temperature-independent pitch is possible to avoid "reverse twist". It has furthermore been possible to show that compensation of the temperature drift of the threshold voltage of a twisted nematic cell can be achieved if the pitch of the helix decreases as the temperature increases (P. R. Gerber, Physics Letters 78A, 285 (1980)). The same applies to the phase transition effect, in that compensation of the threshold voltage drift is achieved by a helix pitch which decreases greatly as the temperature increases (A. Gobl-Wunsch, G. Heppke and F. Oestreicher, Journal de Physique 40, 773 (1979)).
The liquid crystal phases used for these purposes in general consist of mixtures of non-chiral liquid crystal compounds, to which chiral compounds are added to produce the helix structure. Virtually all the known chiral doping substances induce helix structures, the pitches of which increase over wide ranges to a greater or lesser degree as the temperature increases. Only certain spiro-biindane derivatives with a negative gradient of the temperature function are reported in the literature (Advances of Infrared and Raman Spectroscopy 8 (1981) Chapter 4). In practice, however, it was not possible to eliminate the troublesome temperature drift with these compounds. It has hitherto been possible to achieve the negative gradient in the temperature function frequently desired only by using two suitable doping substances with a different sense of rotation and a different relative temperature dependency (German Patent A No. 2,827,471). The disadvantages of this multiple doping process are, inter alia, the observance of the exact concentration ratio of the two chiral compounds, the restriction to a limited temperature range and the high total concentration necessary for the doping substances (A. Gobl-Wunsch, G. Heppke and G. Oestreicher, Journal de Physique, 40, 773 (1979)). External applications are therefore not possible.